In DMFCs, a crossover phenomenon in a fuel from an anode side to a cathode side is significant. Since crossover causes not only loss of a fuel but also decrease in the potential of a cathode, it is desirable to repress crossover as possible. In order to increase the properties of DMFCs sufficiently, it is also demanded that an anode catalyst layer has high diffusability of a fuel.
In order to improve fuel diffusability, optimization of the pore structure of an anode catalyst layer is suggested (see, for example, JP-A 2001-338651 (KOKAI) and JP-A 2005-26174 (KOKAI)). For example, a pore diameter is controlled. As techniques for improving the pore structure of a catalyst layer, changing of a carrier material, incorporation of a different carrier material, a water-repellent or hydrophilic treatment, and introduction of a pore forming agent, and the like are present, but all of these techniques are not sufficient. Even if a large amount of a catalyst is used, demanded properties cannot be obtained.